Contact heaters for quartz crystals in evacuated enclosures

ABSTRACT

Thin metallic films are deposited on surfaces of quartz crystals in selected patterns and appropriate leads are provided to pass a current through the metallic film in order to heat same and thereby heat the crystals. The crystals are mounted in normal manner to an evacuated chamber. By use of the thin film contact heaters, temperature equilibrium is achieved at a much faster rate of time, with a higher rate of stability and with substantially reduced power consumption.

[ 1 Feb. 6, 1973 United States Patent 1 Bloch Z T R A U m r A U RC A V EEN H m S LU C S TO TSL NYC ORN CCE W 3,431,392 3/1969 Garlandeta1............... ........219/210 3,413,438 11/1968Gardneretal................ ......219/210 Primary Examiner-C. L.Albritton [75] Inventor: Martin Bloch, New York, N.Y. Atmmey AmSter &Rothstein Assignee: Frequency Electronics, lnc., New

[57] ABSTRACT Thin metallic films are deposited on surfaces of quartzHyde Park, NY.

crystals in selected patterns and appropriate leads are provided to passa current through the metallic film in order to heat same and therebyheat the crystals. The crystals are mounted in normal manner to anevacul 7 9 l 9 1 7 3 A1 0 .mL P MD. FA 11. 21 22 ated chamber. By use ofthe thin film contact heaters, temperature equilibrium is achieved atamuch faster rate of time, with a higher rate of stability and withsubstantially reduced power consumption.

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1 Claim, 8 Drawing Figures [56] References Cited UNITED STATES PATENTS3,201,621 8/1965 Milner..............................219/2l0X 1 CONTACTHEATERS FOR QUARTZ CRYSTALS IN EVACUATED ENCLOSURES The presentinvention relates generally to construetions for piezoelectric devicesand specifically the means for achieving direct and improved temperaturecontrol of piezoelectric devices such as quartz crystals,

Such crystals are known to be temperature sensitive and therefore theyhave been traditionally placed in fine tolerance ovens which maintaintheir temperature to a high degree of accuracy, thereby to stabilize theoscillation frequency of the crystal. Although such devices have beensuccessful in the past, their construction, as known to date, results inproducts which are larger than desired, which have power consumptionsgreater than desired and which display time periods for reachingequilibrium temperature longer than desired.

Investigations have disclosed that a variation in the manner in whichheat is introduced can substantially improve the performancecharacteristics of these devices. Rather than by use of the conventionalovens known to date, it has been determined that a system of thin filmdeposits, directly on the crystals, can be utilized as electricresistance heaters to produce substantially improved results.

Accordingly, it is an object of the present invention to provide new andimproved means for heating piezoelectric devices. More specifically, itis an intended object of the present invention to provide a structure inassociation with a piezoelectric crystal, such as a quartz crystal,wherein the crystal is directly heated by an electrical resistanceheater comprising a thin film deposited on the crystal.

It is further within the contemplation of the present invention toprovide a piezoelectric crystal device in which an equilibrium operatingtemperature can. be achieved at an improved rate and stabilizationachieved and maintained with low power consumption.

It is still further within the contemplation of the present invention toprovide such a device with an efficiently operating heater which isinherently free from impurities which might migrate into the crystalmaterial. I i i 1 It is finally and generally an object of the presentinvention to provide an improved quartz crystal device in an evacuatedenclosure.

In accordance with one presently preferred embodiment of the invention,there is provided a piezoelectric oscillator device which includes avessel in which is formed an evacuated chamber. Mounted within thechamber is a piezoelectric crystal-having electrodes for exciting thecrystal and lead wires attached to those electrodes and extendingoutwardly'through the walls of the vessel. Means for controlling thetemperature and temperature stability of the crystal are provided andcomprise a thin film, vacuum-deposited, electrically resistive heaterelement in intimate engagement with and upon the surface of the crystal.The heater element is generally patterned to follow the periphery of thecrystal itself. Lead wires are operatively engaged with the heaterelement to provide a sourceof current to flow through the heater elementand those lead wires extend through the walls of the vessel to theoutside to be connected with a source of power. Power control means arealso provided to govern the flow of power through the heater elementandinclude temperature detailed description when taken in conjunctionwith the accompanying drawings wherein:

FIG. 1 is an exploded elevational view, with portions 1 broken away forthe sake of clarity, of a complete piezoelectric electric device made inaccordance with the present invention including an evacuated chamber,

lead pins extending through the walls of the chamber, a crystal mountedwithin the chamber and contact heater means engaged with the crystal;

FIG. 2 is a side sectional view of the device shown in FIG. 1 in itscompletely assembled configuration;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 1 looking inthe direction of the arrows and is, therefore, a plan view of thecrystal;

FIG. 4 is a sectional view taken along the line 4--4 of FIG. 3 lookingin the direction of the arrows;

FIG. 5 is a sectional view taken along the line 55 of FIG. 3 looking inthe direction of the arrows;

FIG. 6 is a partial view similar to that of FIG. 3 showing an alternateaddition of a temperature sensing device such as a thermistor with itsappropriate lead wires;

FIG. 7 is a schematic of a control circuit in which the variation inresistance of the heater as a function of 2 temperature is utilized in acontrol circuit for a delivery of power to the heater; and

FIG. 8 is a schematic diagram of a control circuit for power delivery tothe heater dependent upon variations and resistivity of an externaltemperature sensing device such as a thermistor.

' Referring now to FIGS. 1 and 2, there is shown a precision oscillatordevice generally designated by the numeral l0. The device utilizes anevacuated vessel 12 formed by a metallic base 14 and metallic cap 16which provide an internal chamber 12a. Specifically, the base 14 has askirt and shoulder which conform to the skirt and internal diameter ofthe cap 16 such that the vessel 12 can be evacuated and the two halvesjoined together withan airtight seal. Mounted within the chamber 12a isa piezoelectric device generally designated by the numeral 18. Thedevice in this illustrative embodiment is a piezoelectric crystal ofplanar disc shape. It is provided with standard electrodes or contacts20, 22 for impressing the excitation voltage thereon and it is mountedwithin the chamber 12a by means of lead contacts 24, 26 attached to theelectrodes 20, 22 respectively. The lead contacts 24, 26 extend to thecontact pins 28, 30 which pass through airtight openings in the bottomwall of the base 14. In the usual fashion, a layer of ceramic material32 is provided to insure an airtight seal at the point where the leadwires pass through the wall of the vessel 12. The foregoing is generallydescriptive of the prior art as well as the environment of the presentinvention.

- resistance very narrow limits even where there are significantvariations in the ambient temperature. In order to heat the crystal l8,and in accordance with the present invention, there is formed around theperiphery of the crystal 18 a direct heater structure 34, 36, theconfiguration of which is best seen in FIGS. 3, 4 and 5. One

portion of the heater is applied on one surface of the crystal I8 andanother portion is applied on the opposite side. Both portions of theheater are applied adjacent the periphery of the crystal l8 and extendaround the greater portion of the edge of the device.

When a source of direct current is caused to flow through the heaters,they heat up and thus raise the temperature of the host crystal 18. Ithas been found that these contact heaters operate most efficiently whenarranged in patterns substantially as shown in heaters in parallelcircuitry. Elimination of some of the interconnecting arms woulddecrease the number of parallel branches and would thereby increase theresistance of each individual branch. Alternatively, the heaters couldbe arranged in a series circuit thus increasing the total resistance asseen from the heater lead pins 42, 44. The preferred value of theresistance is a design function and the disclosed structure allows foras wide a variation as possible. It has been found that resistancevalues in the range of 300 ohms provide good results.

As may be best seen in FIG. 5, the heaters 34,36 are applied directly tothe surface of the crystal l8 and, in this embodiment, are of the shapeof an incomplete circular band immediately adjacent the periphery of thecrystal. As shown in FIG. 4, the connecting arms 38 are integrallyformed with the heater elements 34, 36. The material of the heaters 34,36 and the connecting arms 38 may be selected from a large variety ofmetallic elements and alloys or any other material which can be formedinto a thin layer and which will produce heat when traversed by electriccurrent. It has been found that nickel and platinum both perform wellfor this function but that aluminum does not react quite as well becauseof unsatisfactory results Withregard to heater stability. The heatersare preferably deposited by conventional vacuum depositing of thinmetallic film. Vacuum deposition is, of'course, a well known procedurein this technology.

The structure of the entire device 10 is designed to minimize heat loss.Convention losses through space around the crystal I8 are'virtuallyeliminated because of the evacuation of the chamber 12a. Radiationlosses are extremely small because of the polished interior surfaces ofa base 14 and cap 16 of the evacuated vessel. The majority of any heatlosses which do exist occur through the various le'ad wires'and, tominimize these losses, those lead wires are made as thin and long aspossible consistent with electrical and mechanical requirements.

Means are provided within the crystal device 10 to control the flow ofelectrical power to the heaters 34, 36. In some relatively rareinstances, the only control necessary is a preset, non-varying controlwhich determines the amount of power which fill flow into the heater.For example, in an installation to be operated under a large body ofwater where the ambient can be considered to be essentially of zerotemperature variation, the power input can be designed to hold thetemperature of the crystal at the desired operating temperature with dueconsideration being given to heat losses at the equilibrium state.However, in most instances, there will be significant ambienttemperature variations, or other parameters such as a requirement for afast heat up capacity, and thus means are provided to continuously sensethe temperature of the device and responsively vary power input to theheaters. Temperature sensing means are employed within the device tocontinuously measure the temperature and the information that is derivedis used to control the amount of power delivered to the contact heaters.In FIG. 6, there is shown, in schematic form, a thermistor 46 located onthe heater immediately adjacent its connection to the heater lead wire40. The thermistor can be placed at a variety of locations and it hasbeen found that it is advantageous to place it on the heater adjacentthe heater lead wire or on the heater lead wire itself in order to makeit sensitive to heat losses as they occur by conduction along the leadwires. Appropriate lead wires extend from the thermistor 46, one ofwhich goes to the pin 48 and another to the pin 50. The pin 48 is shownin FIGS. 1 and 2 as being a vessel grounding pin and the other pin 50may either be the same pin as one of those alreadydescribed or may be anadditional pin passing through the wall of the evacuated chamber. Itwill be appreciated, of course, that the showing in FIG. 6 of thethermistor and its electrical and mechanical connections is schematic innature.

FIGS. 7 and 8 present two different schematics of the control circuitryin which power to the contact heater is controlled as a function of thetemperature in the device. In FIG. 7 a circuit is shown in which thevariation and resistivity of the contact heater itself is used as thetemperature sensing element and in FIG. 8 the schematic depicts the useof a device such as the thermistor 46.

In the schematic drawing of FIG. 7, there is shown a heater 34, 36arranged in a four-arm bridge along with small temperature coefficientresistors R1, R2 and R3 which may be, for example, Corning Glass C styleresistors which have a temperature coefficient of 0. 1 X10 "/C. Theheater 34, 36 may be either platinum or nickel, the temperaturecoefficients of which are given as 3.9 l0' /C and 6. l 7Xl0'/C,respectively. The differential amplifier reads the voltage differencebetween the centers of the two branches of the bridge and, through thetransistor T, controls the heating power delivered to the heaters.

The schematic of FIG. 8 is similar to that of FIG. 7 except that ratherthan utilize the heater material as the temperature sensing device, athermistor 46 is utilized Experimental details utilizing the concepts ofthis invention have been given in a paper entitled The DirectTemperature Control of Quartz Crystals in Evacuated Enclosures by Tinta,Matistic and Lagasse which was published at the frequency and timecontrol symposium sponsored by the U. S. Electronic Command during theweek following Apr. 25, 1970. The data and disclosure of that paper maybe referred to for further details and it is incorporated here byreference.

It will be appreciated that in accordance with the present inventionthere is provided a mechanism for the direct introduction of heat to apiezoelectric device. Rather than by heating an oven to indirectly heata crystal, the crystal itself becomes the element heated through directcontact with a electrically remaintain critical operating temperaturewith variations in ambient temperature far more reliably at much lesspower consumption than were available in the prior art.

Only one specific configuration for a thin film contact heater has beenillustrated. It will be obvious to those skilled in the art that avariety of geometries for contact heaters as disclosed herein may beused on a variety of crystal configurations.

What I claim is:

1. In a piezoelectric resonator of the type having a piezoelectriccrystal mounted within an evacuated chamber through the walls of whichsaid crystal is supplied with excitation electrodes, the improvementcomprising: a thin film, metallic, electric resistance heater formed onand in intimate contact with a portion of the surface of said crystal;means for sensing changes in temperature of said crystal; electric powermeans for delivering a source of current to said heater; and con trolmeans responsive to said temperature sensing means for controlling thepower delivered to said heater dependent upon the temperature of saidcrystal; said heater being formed of a positive coefficient resistancematerial and said temperature sensing means includes a resistance bridgewhich utilizes said positive coefficient resistance heater as oneelement thereof.

